Optical scanning is usually accomplished by one of two simple mechanical means. These are, a moveable mirror which produces a scan angle, which is twice the mirror movement angle, and a moveable prism which provides a similar but non linear beam scan function. Optical energy from an optical source impinges on the mirror or the prism and is redirected as the moveable member oscillates. Both of these methods require a moving optical element to accomplish the angular beam scan, which results in inherent limitations on upper scan frequency as well as suffering from the effects of friction and wear. A description on prisms as dispersors is found in Williams and Becklund, "Optics, a Short Course for Engineers and Scientists", Wiley-Interscience 1972, pages 251-254.
FIGS. 1 and 2 show a typical circuit for and the phenomena associated with the inverse Stark effect. The inverse Stark effect utilizes absorption spectra instead of emission spectra as in the Stark effect. The apparatus in FIG. 1 is comprised of a line spectra optical source 10, such as a laser. The output light beam 12 traverses Stark cell 14 and emerges as output 16. The Stark cell is filled with an optical medium which has an absorption line at the frequency .nu..sub.0, near the source emission frequency at .nu..sub.1. Closing switch 18 applies voltage from battery or power supply 20 to capacitor plates 21 and 22, thus producing an electric field across the Stark cell. This electric field splits the absorption line of the cell medium and the resulting shift in absorption frequency is typically used to absorb the input light and thereby amplitude modulate the output light. Switch 18 may obviously be replaced with an electronic switch to provide higher frequency modulation. This prior art is well known to those technically versed in the field.
FIG. 2 illustrates the physical phenomena of the inverse Stark effect. The Stark cell medium 14 possesses an absorption line spectra, one line of which is shown at A. The index of refraction, .mu., near this absorption line is shown at B; the negative slope portion of this curve is known as anomalous dispersion. In an electric field, this absorption line of A may be made to split as shown in C. In general, the stronger the applied voltage the wider the separation of these two absorption lines. The index of refraction is also altered as shown at D. The symbol .nu. represents the optical wave number (proportional to frequency). Anomalous dispersion and the Stark effect are described in Jenkins and White, "Fundamentals of Optics", McGraw-Hill, 1957.
As shown in FIG. 2, if light of frequency .nu..sub.1 or .nu..sub.2, is provided by source 10 and Stark cell 14 has the absorption lines of A and C in the absence and presence respectively of an electric field, strong amplitude modulation is produced at output 16. Prior art provides for mechanical beam scanning with moving elements such as mirrors or prisms and for amplitude modulation of narrow spectral sources using the tunable selective absorption of Stark cells.